3 weeks in laos - which route to take?

Not sure why you should want so much time in Vang Vieng and Luang Prabang.
Perhaps take a day from one of them and add to Vientiane. Vientiane has at least two days of sights: Putaxai, Great Stupa, the waterfront walk, wat si wisaket, wat si muang, the national museum and getting the #14 bus from Talat Sao (Morning market) to Xieng Kuang (the Buddha Park)

Yep, swap some of the Vang Vieng time for Vientiane time. Otherwise pretty good. Luang Nam Tha is great.

I was under the impression that there's pretty much to see and to do in VV and LP - kayaking , caving , day treks to villages around etc , 3 days for VV is really too much?

I would keep it pretty much as it is, but take a day or two off VV and one day off LP to add to either the NK/MN area, or Vientiane.

I live in Vientiane, and whilst I personally wouldn't get too excited about the standard "sights", it certainly has an original atmosphere and some good bars & restaurants to wander round which you won't be able to do in a single day.

Looks good.
A couple of points:
On our third visit to LP, We managed to enjoy 7 days there. soak it all in.
Don't miss eating in the Night Market.
Nong Khiaw / Muang Ngoi are often criticised by OP, but we love the aera. +.
Plenty of great walks to other villages.
Return to L.P. by boat, stunning. One of the Great Journeys one can experience. Bring food. When your having breaky, get the cafe' to do you a lunch-pack.
Seek out roof top bars in Vientane, for the Mekong sunsets, in the late arvo.

Hi
you don't say how you are traveling around Laos. You could save some time by going to Nong Khiaw from Luang Namtha - it's on the way back to Luang Prabang. From Luang Namtha to Oudomxai to Pakmong then it's just a short hop from there to Nong Khiaw. Have fun, it's a beautiful place. We're planning our 3rd trip!

I like your original itinerary, but allow yourself flexibility along the way. You might want to spend more time in LNT for instance and less in the more southerly places. Nice thing about Lao is that you can wing it. BTW, DrMoss' idea to go directly from LNT to Nong Khiaw is good. Another interesting route would be to head from LNT to Muang Khua by bus, then boat down to Ngoi and Khiaw and LP. It may seem blasphemous to some, but I much more enjoyed Lao N of LP than I enjoyed LP proper.

Thanks everybody !
Got another question

In Luang Namtha I'm looking for spending about 2 days in the NPA which would leave me 3 days for the villages located around Muang Sing(Muang Long aswell perhaps)

Question is - are the villages around MS doable independently(bicycle+map) or taking an organized trek through an agency is a must? also I would like to travel along and watch the Chinese border how are the sights around there and is there an easy way to reach it?
About Muang Long - I see there isnt much info about the town any info from travelers who've been there ?

Also I wanna do a half day/full day in LP for visiting the Wats and I see there's 32 of them , which are the best to see/visit?

Thanks in advance !